A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to monitor the lithographic process, it is desirable to measure parameters of the patterned substrate, for example the overlay error between successive layers formed in or on it. There are various techniques for making measurements of the microscopic structures formed in lithographic processes, including the use of scanning electron microscopes and various specialized tools. One form of specialized inspection tool is a scatterometer in which a beam of radiation is directed onto a target on the surface of the substrate and properties of the scattered or reflected beam are measured. By comparing the properties of the beam before and after it has been reflected or scattered by the substrate, the properties of the substrate can be determined. This can be done, for example, by comparing the reflected beam with data stored in a library of known measurements associated with known substrate properties. Two main types of scatterometer are known. Spectroscopic scatterometers direct a broadband radiation beam onto the substrate and measure the spectrum (intensity as a function of wavelength) of the radiation scattered into a particular narrow angular range. Angularly resolved scatterometers use a monochromatic radiation beam and measure the intensity of the scattered radiation as a function of angle.
As mentioned above, a pattern is created on the surface of a substrate, this pattern representing the IC, or whatever product is being made on the substrate. The way that the pattern is made is that repeated layers of resist are laid on the substrate, then exposed, then washed or baked or other such post-exposure processes. It is desirable that each of the layers that is to be exposed be aligned with the layer below it such that the pattern builds up as accurately as possible, ensuring effective electrical connections where required and also enabling smaller and smaller products to be created while avoiding cross-talk between neighboring structures. Scatterometers are useful in determining whether or not subsequent layers are aligned as they should be. The alignment of subsequent layers is known as overlay. An error in overlay means that a layer is offset with respect to a layer below (or indeed above) it. If a layer is offset with respect to a layer below it, the quality of electrical contact between structures within these respective layers will be reduced. In extreme cases, complete loss of electrical contact or short-circuiting might occur. The same is true if a product layer is rotated with respect to the product layer below it. These types of overlay error may be measured using scatterometers.
In order to measure and thereby correct for (or, preferably, prevent) overlay errors, test patterns are created on substrates which have known properties and which are tested using a radiation beam and a scatterometer detector, which measures the diffraction spectra of the radiation beam that has reflected from the test pattern. In order to reduce the amount of space taken up by these test patterns, the test patterns are generally created in the scribe lanes between dies and fields on the substrate. It is these scribe lanes that will subsequently be sawn in order to separate the various products, and so are not useful for product. The test patterns are generally known as overlay measurement targets or overlay targets. However, the scribe lanes are small (to leave more room for product) and are also generally packed with a large range of test patterns for various purposes. Because of the relatively large size of the overlay targets, for on-product overlay measurement (i.e. measurement on substrates that also contain product, as opposed to test substrates that may be used purely for testing), in practice, the targets are in the scribe lanes between the dies. However, this means that models are used to predict real product overlay because the real product overlay is not being measured directly.
Models that are used in interpolating scribe lane overlay targets to in-die product are approximations. These approximations may have some errors. The errors may be made worse if fewer targets are available in the scribe lane. It is often the case that fewer than an optimum number of overlay targets are available because the scribe lane “real estate” is valuable to users of lithographic apparatuses (as mentioned above).
To minimize above-mentioned interpolation errors, it is desirable to be able to measure overlay on in-die targets. Because substrate “real estate” is at a premium inside the dies, it is desirable that these overlay targets be as small as possible, and preferably 10 by 10 microns or less. However, with very small targets, there are stringent size requirements which may lead to signal-to-noise ratio problems. Furthermore, signal-to-noise ratios and cross-talk between the overlay target and neighboring product structure may be undesirable. This is because the illuminating beam will be limited to the size that it can be focused down to, which may be larger than the target itself because of the desired extremely small target size.